Flow rate sensors are also known as volume sensors. Generally, they take the form of displacement meters. Examples of these are gear sensors, screw spindle meters, oval-wheel meters, cylindrical-piston meters or alternatively measuring turbines or proportioning gear pumps. They are used to measure a volume, a throughflow quantity or the rate at which a medium, here therefore a fluid, passes through the measuring instrument. The fluids may be liquids, pastes or gases.
In practice, flow rate sensors are often not measuring instruments in the narrower sense because the evaluation electronics are not part of the instrument but situated externally. Nevertheless, the term “flow rate measuring instrument” is often used and reference is also made to measuring chambers and measuring mechanism elements etc. The flow rate sensors are frequently also described as volume sensors, throughflow sensors, flow rate measuring instruments etc.
The volume sensors or flow rate sensors merely sense the throughflow or a throughflow volume and supply a signal to this evaluation unit or evaluation electronics, which only then produce a measured value therefrom. The expression “flow rate sensor” is used below. A confusion with specific structural elements in the instrument that are detectors or sensors in the narrower sense is avoided by use of the full designation.
A common feature of most flow rate sensors or volume sensors is that they scan the movement of a rotating gear wheel. In gear sensors, for example, two mutually meshing gear wheels are mounted in a freely rotatable manner. A medium (generally a fluid, such as a liquid or a gas) is fed to the two gear wheels, namely to the region where they are in mesh. The medium therefore passes into the chambers that are formed mutually in the tooth spaces of the two gear wheels. As a result of the following flow of the medium, the quantities situated in the chambers of the gear wheel are conveyed from the inlet side to the outlet side and by means of the movement of the teeth then set the gear wheels in rotation. The two gear wheels in said case rotate in opposite directions. In the housing surrounding the gear wheels a magnet is disposed, which builds up a magnetic field. This magnetic field is influenced by the rotating gear wheels. These changes of the magnetic field may be scanned by means of one or more suitable sensors.
In this case, each tooth of the gear wheel that passes under the sensor or sensors leads to a scannable pulse. From the number of these pulses it is then possible to determine the angle, through which the gear wheel was rotated, and/or how many revolutions the gear wheel has completed altogether in a specific period of time. From this information it is then possible to draw a conclusion about the quantity of fluid or of some other medium that has flowed through the flow rate sensor, or to determine the rate of flow of the fluid. Naturally, it is possible conversely, in terms of closed-loop control, to define in a closed-loop control circuit a specific setpoint value for the quantity of the medium to be conveyed or for the rate of flow, which the flow rate sensor together with a pump then adjusts accordingly.
For some time now, such instruments have been successfully marketed and are already known for example from DE 25 54 466 C3. There is in particular a demand for flow rate sensors, which are also capable of extremely precise measurement of small delivery quantities of the medium to be delivered and/or which allow an extremely precise indication of the delivery rate. The smallest measurable quantity of the medium to be delivered consists of the quantity corresponding to a rotation of the gear wheel through the angle that exists between two teeth of the gear wheel. This would correspond to the interval between two counted pulses during a rotation of the gear wheel. This volume then also determines the inaccuracy of measurements in the case of large quantities. It is also known as the “tooth space volume”. It is also an indication for the overall size of a flow rate sensor.
If for example a quantity has been delivered with 9 such pulses, then the measuring instrument is unable to conclude which proportion of this tooth space volume or minimum volume has flowed through the measuring instrument before the first pulse and after the ninth pulse. The value “9 pulses” therefore stands for a delivered quantity, the volume of which is between slightly more than 8 and almost 10 of these minimum volumes. In the case of small delivery quantities or very slow delivery rates and equally in the case of rapidly changing delivery rates, this is quite a considerable measuring inaccuracy.
This problem has also already been identified and in this respect for example in U.S. Pat. No. 4,641,522 it is proposed that the sensor be disposed such that the movement of both gear wheels may be utilized for measurement. A similar proposal is made in EP 0 741 279 B1, with EP 0 642 001 A2 even proposing a circular arrangement of a complete ring of sensors in order to be able to carry out as many measurements as possible.
The aim of all of these proposals is, by increasing the number of measurements or measuring facilities, to reduce the minimum volume of the relevant medium that is to be delivered between two pulses and hence improve the measuring accuracy. The drawback of all of these arrangements is a considerable build-up of equipment as a result of the additional sensors and the incoming lines needed for them.
Further factors that also have to be considered are that occasionally and depending on concrete actual requirements the measurements also have to be carried out on relatively hot, flowing fluids or gases, that a high outlay for sealing of the instruments is necessary because these fluids or gases may be toxic, flammable or expensive, and that it is quite possible for the fluids and gases also to be aggressive media. All of these factors occasionally add considerably to the complexity and cost of the corresponding equipment.
An attempt is also made to achieve this mechanically by using gear wheels that are as small as possible. However, this then has the drawback that no meaningful processing of large quantities or rapidly flowing fluids, in particular gases, is possible with such measuring instruments.
The object of the present invention is to propose a practicable solution for a flow rate sensor, with which the measuring accuracy may be improved with a lower outlay for equipment.